The crystallization of glass is generally performed by external heating using an electric furnace or the like or by direct heating through laser irradiation or the like. For example, Japanese Patent Application Laid-Open Publication No. 2006-83044 discloses a method for forming a crystalline phase containing a rare-earth element by irradiating the surface of glass containing the rare-earth element with an ultra-short pulse laser.
Applications of microwave irradiation to various material processing have been studied, such as inorganic material synthesis, and sintering, joining and crystallization of ceramics. These applications utilize a self-heating phenomenon caused by loss of electromagnetic energy penetrating the inside of substances. Heating by microwave irradiation is a thermal processing requiring shorter time and less energy compared to general external heating because it generates heat only in a reactive component including oxides of transition metals, semiconductor substances and metallic conductive substances which exhibit microwave absorption.
Japanese Patent Application Laid-Open Publication No. 2002-186849 discloses that a synthetic precursor liquid for various inorganic materials is prepared and is continuously injected into a tube-shaped microwave reactor to be synthesized and crystallized, thereby reducing the time required in the crystallization process to a few minutes to a few tens of minutes, unlike a conventional hydrothermal method which requires longer time in the crystallization process.
Japanese Patent Application Laid-Open Publication No. 2005-294744 discloses a method for producing a photocatalyst made of anatase titanium oxide by irradiating amorphous titanium oxide with ultraviolet or visible rays having energy equal to or larger than the band gap and with microwave to heat and crystallize the amorphous titanium oxide.
When glass is crystallized by conventional external heating through heat conduction from a heat source and direct heating by laser irradiation or the like, all crystals in a thermodynamic equilibrium state are precipitated, causing a problem that even an unnecessary crystal is precipitated.
The present invention provides a thermoelectric conversion material in which only a desired crystal is selectively precipitated.